Macbook Pro 2010 no audio output

Recently my 2010 Macbook Pro's internal speakers stopped working. Whenever I try and use the keyboard to change the speaker volume, the screen comes up with the speaker image, but instead of the scale appearing below as usual, it produces a stop sign (circle with a line through it - I've attached a screenshot). Moreover, I can't change the volume in the bar at the top of the screen. Does anyone have any solutions? I've already been into system preferences, and the speakers are selected.